The present invention relates to a flat tape intended for the manufacture of one layer of a flexible pipe for subsea transport of hydrocarbons and gases, especially an unbonded flexible pipe of the offshore type as defined in the API 17B and 17J recommendations of the American Petroleum Institute.
Such pipes are formed from layers—partly metallic and partly polymeric—that are not bonded together. These pipes have at least one polymeric pressure sheath surrounded by a pressure vault, at least partly produced by the helical winding of elongate metal elements leaving a slight clearance between their turns called the “inter-turn gap” or “gap” (generally at least one layer of elongate metal elements, especially profiled wires, wound with a short pitch) and one or more plies of reinforcing elements wound with a long pitch.
One of the problems encountered in practice is creep of the pressure sheath into the gaps existing between the turns of the pressure vault. This creep takes place owing to the combined effect of the internal pressure and of the temperature of the fluid transported in the pipe. Depending on the effluent and its temperature, the plastic sheaths used may, for example, be produced from materials of the polyvinylidene fluoride PVDF type (for example the coatings known under the brand names Coflon® and Gammaflex®, for applications at relatively high temperatures, typically above 80° C.). These materials have a tendency to undergo relatively substantial creep into the inter-turn gaps of the vault. This creep leads to the formation of creep pips in the gaps.
In dynamic applications, such as in risers, these pips are a source of fracture initiation because of the movements of the vault turns which tend to shear these pips.
One solution commonly used for trying to solve the creep problems is the production of anticreep layers as described in document FR 2 743 614, which covers the specific end-fitting of such pipes. These layers are expensive in terms of raw materials, but also in terms of production costs.
Another solution envisioned for solving this problem could consist in using a conventional tape wound with a short pitch around the pressure sheath, the tape being intended to prevent said sheath from creeping. However, this solution is excessively difficult to implement for the following reasons.
The overlapping of a tape with a unidirectional fiber during short-pitch taping would induce an overstress in the fibers at the overlapping part, resulting in substantial deterioration in the fibers, extending as far as the fracture at the edge of the tape. Such a fracture would then cause the tape to be destroyed, which would no longer fulfil its anticreep function. To avoid these overlaps, it would be necessary to lay the unidirectional tape edge to edge, that is to say without a clearance. However, to wind a tape without a clearance is virtually impossible because of the following reasons: the precision of existing tape winders (pitch control, tension variation in the tape laid, resulting in a variation in tape width), the geometry of the pipe (tolerance on the diameter) and the sag of the pipe (slightly domed shape of the pipe under its own weight during the taping operations in the tape winder), etc.